1. Skin Barrier Integrity and Microbiome-Mediated Immune
Regulation and Dysregulation
Author: Asad Zaidi
Capstone Advisor: Mary Montgomery
5/6/2015

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ABSTRACT
Our skins are home to a whole host of commensal microorganisms. Since the skin is
the organ most in contact with the outside environment and the first immunological barrier,
the skin commensals help out by regulating both innate and adaptive immunity. However,
barrier defects in the skin can dramatically change the behavior of the microbiome from
protective and benign to inflammatory and pathogenic, leading to conditions such as atopic
dermatitis.
PREFACE
Eczema/atopic dermatitis is something that runs in my family. It is a condition
associated with immunity and the immune system. While I was initially looking for a
topic for my final paper for Immunology, I looked for mechanisms surrounding eczema.
We had just covered the gut microbiome in class and the concept fascinated me, so when
I discovered that eczema was also connected to the microbiome, I began my research.
The fact that studying the microbiota is so recent – I was literally finding a freshly
published article on the topic every few days – also kept my interest piqued; there is still
so much to discover!
BLURB
Maintenance of skin barrier integrity is key for positive regulation of skin
immunity by commensals. Compromised integrity can lead to dysregulation of skin
immunity by commensals and, ultimately, atopic dermatitis.

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INTRODUCTION
One classic scientific fact used in many Biology classes to shock, amaze and
intrigue students is that roughly ninety percent of the cells that constitute the human body
contain microbial, rather than Homo sapiens, genetic material [1]. Although this idea has
quickly become assimilated into the mainstream body of ‘general knowledge’ this finding
is actually quite recent. It was not until the recent advances in metagenomics technology
and the rise of low-cost, high-throughput genetic sequencing techniques – techniques
such as 16S gene profiling (Fig 1) – that the realm of microorganisms living inside and
on the human body could extensively be explored and studied [2].
The Human Microbiome Project was launched in 2008 in an attempt to sequence
and profile all the microorganisms commonly found thriving alongside human cells in the
living human body [1]. What it discovered was a stunning array of microbial diversity
(Fig 2).
Such a staggeringly large number of non-human cells cannot simply be
bystanders in the human body, neither affecting nor being affected by the many bodily
functions and homeostatic processes occurring every instant. The microorganisms –
mostly bacteria of the four phyla Actinobacteria, Bacteroidetes, Firmicutes and
Proteobacteria along with viruses and fungi – that make up the microbiome [1] are both
specific and highly specialized for inhabiting the region of the body in which they are
found.
Niche specialization in a host organism involves not only being adapted to the
environment, but also being an asset for that particular bodily niche, so that there is an
incentive to being kept around. Thus, many microbes that are part of the microbiome are

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described to have commensal relationships with their hosts, that is, a relationship between
two organisms where one of the two benefits from the relationship while the other
remains unaffected. However, it could be argued that, given the protection conferred by
certain prominent species that will be discussed in greater detail in this paper, some
relationships could be characterized as mutualistic, where both co-existing parties benefit
from the relationship [3]. Despite this, organisms that constitute the skin microbiome will
be referred solely to as ‘commensals’ in this paper to reflect the scientific community’s
consensus on how to label them.
As a large proportion of our bodies’ resources are spent on immunity, immune
function and modulation, it is logical for commensal microbes to ‘earn their keep’ by
initiating, regulating, catalyzing or assisting with these very processes. Indeed, the gut
microbiome has already been established as a key player in the immunity of the whole
body in general and in the gastrointestinal tract specifically [4]. The microbiome that has
only just begun to garner attention is the human skin microbiome.
THE SKIN MICROBIOME
The skin is the largest organ of the human body. It functions as a physical barrier,
forming the initial immunological threshold that both prevents the entry of pathogens and
potentially harmful substances as well as the loss of water and essential solutes [5]. It is a
complex, multi-layered surface riddled with invaginations, glands and hair follicles, all of
which provide a multitude of habitable niches with varied thickness, moisture content and
follicular density. It is estimated that every square inch of human skin is inhabited by
approximately 1 billion bacteria [6].

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The microbiome of the human skin begins developing not long after birth. While
the skin of the fetus is considered to be mostly sterile in its mother’s uterus, passage
through the birth canal brings newborns into contact with bacterial communities that will
inevitably go on to form colonies. Domniquez-Bello et al. show that the mode of delivery
impacts the identities of bacteria that form the initial microbiome. This, in turn, also
impacts the health of the baby [7]. Neonates delivered vaginally have skin microbiomes
that resemble the vaginal microbiotas of their mothers, whereas neonates delivered via a
Caesarean section procedure have skin microbiomes more likely to be made up of the
types of bacteria found on the mother’s skin.
An infant’s skin microbiome is quite homogenous at first, evolving and showing
increasing diversity over time. A study conducted by Capone et al. observed how the
microbiome changed as infants grew. Swab samples were taken from 31 healthy infants,
each falling into one of three age groups based on infant skin maturation properties, and
from mothers who served as adult controls [8]. The study controlled for ethnicity by
using only Caucasian subjects and for variations in baby bathing methods by normalizing
the procedure volunteering mothers were to follow prior to the swab test. At least for the
first year, the microbiome is dissimilar to that of an adult, primarily due to that fact that
infants have more moisture in their skin, allowing the colonization of a proportionally
greater amount of Staphylococci bacteria. However, with age, the microbiome diversifies
and becomes more akin to that of an adult by the time the infant is 12-18 months of age
[8]. Once adulthood is reached, however, the microbiota becomes relatively less subject
to change.

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Elizabeth Grice is one of the leading researchers of the human skin microbiome.
Her lab at the University of Pennsylvania Perelman School of Medicine, Department of
Dermatology has a keen interest in ‘leverag[ing] our understanding of microbiome-host
interactions to diagnose and treat skin disorders’ [9]. Grice et al., looking to profile the
bacterial diversity found in the microbiome of adult skin, discovered that interpersonal
variation was almost the same as intrapersonal variation when it came to bacterial
community membership and structure [6]. Analysis of microbial communities on the skin
of the palm showed that samples taken from either hand of the same individual shared
only 17% of their phylotypes1
, whereas comparisons of samples taken from different
individuals showed only a 13% phylotype commonality [3]. However, microbial
communities at different body sites are almost uniformly dominated by the phyla
Firmicutes, Actinobacteria, Bacteroidetes, and Proteobacteria [6]. Along the way, Grice
et al. made certain discoveries that could have an impact on how studies on the skin
microbiome are conducted. Skin samples acquired by swabbing, scraping and skin punch
were compared. 16S gene profiling of the samples showed that there was a great overlap
between the OTUs (operational taxonomic units) found using each method. This result
indicates that the use of invasive sampling methods is unnecessary. The study also found
a significant degree of comparability between the microbiota present on human skin and
that on murine ear skin, strengthening the case for mice as model organisms for the study
of the skin microbiome. The skin microbiota is relatively stable, with distinct anatomical
niches colonised by specific and specialised groups of microorganisms, as demonstrated
1 Phylotypes are groups that classify organisms based on observable similarities (morphology etc).
These groups do not adhere to taxonomical hierarchy and, thus, the rank at which they describe
groupings (species, phylum etc) can be circumstantially chosen.

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by Grice and a colleague in another study [10]. The composition of the skin microbiota is
more determined by the body site niche (Fig 3) than one’s genetic fingerprint [3].
Examples of distinct body site niches include the glabella (skin between eyebrows
and above nose), the antecubital fossa (the elbow pit), and the interdigital web space (skin
between fingers). Skin regions with a higher density of sebaceous glands have a higher
content of surface lipids and, thus, are inviting habitats for lipophilic bacteria such as
Propionibacterium acnes [3]. This, to some extent, is why certain areas of the skin are
more prone to acne, a condition thought to be associated with the presence of P. acnes.
Microbes not typically found in these niches can usually only exist as transient flora,
temporarily colonising the niches before being driven away due to competition with the
more permanent residents [3].
This and similar findings suggest that the permanent residents of ecological body
niches of the skin have a stake in remaining where they are, but also points towards the
possibility that their existence may be beneficial to their human hosts. The skin
microbiome interacts not only with human skin but also with the environment and forms
a bridge between other microorganisms and human tissue [3].
IMMUNE MODULATION
The outermost layers of the skin are comprised of mostly keratinocytes. These
cells make up the water-proof barrier between bodily tissues and the environment. They,
along with sebaceous glands [11], also participate in innate immunity by secreting
antimicrobial peptides including catheclidin LL37 or beta-defensins. These molecules are
produced against highly conserved pathogenic antigens, typically found on

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Staphylococcus aureus, Group A Streptococcus, and Escherichia coli [3]. Interestingly,
these compounds are not active against Staphylococcus epidermidis, possibly due to the
benefits of hosting S. epidermidis in the skin and resulting from the ‘resolution’ of an
evolutionary arms race between humans and S. epidermidis.
Evidence supporting this hypothesis lies in the production of a polymer called
Poly-N-acetylglucosamine (PNAG) by S. epidermidis. PNAG is primarily an adhesion
molecule that enables the formation of an extracellular biofilm matrix. However, it has
been shown to protect S. epidermidis from innate immune machinery, helping the
bacterium evade neutrophils, immunoglobulin and antimicrobial peptides [12]. Indeed,
this evolutionary arms race might still be underway. PNAG has been shown to have a
stimulatory effect on Toll-like Receptor 2 (TLR-2), a crucial receptor involved in the
activation of innate immune pathways [13]. The paradoxical implication of this is that
molecules that S. epidermidis uses to protect itself from the immune system are, in turn, a
trigger for the immune system. However, experiments utilizing genetic PNAG loss-of-
function mutants are required before contamination by substances with pro-inflammatory
properties can be definitively ruled out. Such contamination has commonly caused
researchers to observe TLR-2 stimulation [13,14].
Commensals and outer skin cells are also frequently in conversation to maintain
healthy immune function and well-being of the skin (Fig 4). Keratinocytes are involved
in the production of free fatty acids, molecules that are key compounds present in human
sebum and are, therefore, extremely common on human skin. Free fatty acids are
produced from the breakdown of lipids such as sebum triacylglycerides. Lipases
responsible for this breakdown come from commensals such as P. acnes and S.

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epidermidis. Certain chain lengths of free fatty acids (C8-C12) have been shown to have
antimicrobial properties against a wide range of primarily gram-positive bacteria [11].
Keratinocytes are not the only cells taking part in innate immune function.
Commensal bacteria, especially gram-positive bacteria such as Streptococcus and
Streptomyces, produce their own cocktails of antimicrobial peptides to inhibit the growth
of other bacteria on the skin and maintain niche dominance [11]. Our knowledge of the
workings of these commensal antimicrobial peptides is limited to the few that have been
isolated and studied, such as Epidermin, Pep5 and epilancin K7, which are most
commonly characterized as compounds produced by S. epidermidis [15].
The modified proteins have a three-ringed structure containing regions that are
both hydrophobic and hydrophilic, enabling disruptive interactions with the microbial
membrane. Similar to the mechanisms by which classic (human) antimicrobial peptides
function, these disruptive interactions create pores in the microbial membrane which
prevent cells from maintaining the balance of their internal environments, cause cytosolic
leakage and can be lethal to the microbial cell [11].
Despite this, little work has been done on understanding these molecules.
Currently, there are no studies looking into the genes in S. epidermidis responsible for
encoding these commensal-derived antimicrobial peptides. However, with the scientific
community’s current keen interest in metagenomics and understanding the human
microbiome, it should not be long before such a study is conducted. Another future
directions that research will need to take before these mechanisms can be deduced are in
vivo studies of skin colonization and competition by knock-out S. epidermidis strains
lacking the ability to produce antimicrobial peptides.

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Additionally, commensals such as S. epidermidis also enhance the innate activity
of keratinocytes. In vivo experiments have shown that S. epidermidis-conditioned culture
medium enhanced keratinocyte activity against potentially pathogenic bacteria by
activating the Toll-like receptor 2 of the keratinocyte. The conditioned culture medium
also enhanced keratinocyte activity against viruses – the keratinocytes were able to better
inhibit human papillomavirus 5 pseudovirus (artificial biological particles with
papillomavirus antigens on their surfaces) survival2
and prevent vaccinia virus plaque
formation in the keratinocyte monolayer [16].
Commensals are also frequently in conversation with the adaptive facet of the
human immune system. A recent murine study has shown that the function of effector or
helper T-cells in the skin is only effectively driven in the presence of resident bacteria
[17]. Helper T-cells steer the function of the immune system by producing and secreting
pro-inflammatory cytokines and are, thus, vital for a robust and balanced immune
response [18]. Commensal bacteria augment the signaling of the cytokine IL-2 in the skin
[17]. IL-2 plays a key role in the pathways that govern and regulate T-cell differentiation.
Therefore, the presence of commensal bacteria in the skin amplifies the local
inflammatory response and encourages T-cell differentiation into effector and memory
cells [17]. The study also found that mice that were raised germ-free generated little or no
memory against skin pathogens and had impaired development of Th17 cells, a subset of
effector T-cells specialized in antimicrobial immunity at epithelial and mucosal barriers.
During a subcutaneous infection of Leishmania major, the absence of S. epidermidis was
observed to have a negative effect on T-cell differentiation [17]. The exact mechanisms
2 Pseudovirus survival is measured based on the number of pseudoviral particles no destroyed or
incapacitated by immune cells.

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that enable S. epidermidis to play a role in immunoregulation are not well understood, nor
is it clear if S. epidermidis is the only commensal that exhibits the immunoregulatory
behavior we see, indicating that replication of these experiments using other common
skin commensals is required.
This evidence gathered so far establishes the benefits of a healthy skin
microbiome. It also provides clues behind mechanisms that may govern the pathogenesis
of inflammatory disorders of the skin.
IMMUNE DYSREGULATION
A number of skin conditions have been linked to immune disruption brought
about by non-normative skin microbiome community membership and behavior. These
include psoriasis, which is linked to unregulated Streptococcus activity; acne, which is
associated with Propionibacterium acnes; and dandruff, which is connected to increased
relative abundance of Malassezia fungus in the scalp. The etiologies of these conditions,
however, has not been thoroughly studied and is not well understood [19]. One condition
that is considered the most common manifestation of asynchrony between the
microbiome and the immune system is atopic dermatitis.
Atopic dermatitis, also known as eczema, is one of the most common chronic
inflammatory diseases of the skin [20]. It is also considered one of the most common
manifestations that result from an atypical skin microbiome. The prevalence of atopic
dermatitis has been increasing dramatically over the past few decades. Today, it is
estimated to have a lifetime prevalence of over 20%, primarily in high-income countries
[21]. Children are more likely to contract atopic dermatitis than adults. The International

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Study of Asthma and Allergies in Childhood, an epidemiological study initiated in 1991
involving over two million children in more than 100 countries over the span of its three
phases, found that atopic dermatitis occurred in between 10 and 20 percent of children
worldwide, with higher prevalence in developed countries [21,22]. It is currently
uncertain why prevalence rates of atopic dermatitis are rising in this trend. The
phenomenon may be linked to the ‘hygiene hypothesis’, the idea that raising children in
an increasingly sterile environment is responsible for reducing immunogenic exposure to
antigens to a rate below that necessary for the regulation of normal immune function
[23].
The pathogenesis of atopic dermatitis includes skin barrier defects, immune
dysregulation and frequent and persistent infection [20]. Since this pathogenesis is a
complex interplay of genetic factors and environmental factors, the scientific community
has been unable to resolve the debate between two hypotheses that may explain it. The
‘outside-in hypothesis’ posits that the dysfunction and dysregulation of the epidermal
barrier is the primary insult, which leads to an immune response. The ‘inside-out
hypothesis’, on the other hand, argues that the disease is mainly driven by cytokines that
cause epidermal inflammation [20]. It is likely, however, that these two hypotheses are
not competing but actually play complicated and complementary roles in atopic
dermatitis.
Batista et al. performed an immunohistochemical study to observe the expression
of IL-17 as well as filaggrin and claudin 1 – skin barrier proteins that work to maintain
the structural integrity of the skin and also prevent transepidermal water loss – in skin
samples from atopic dermatitis patients. Lesional skin in adults with atopic dermatitis

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was shown to have reduced expression of filaggrin and claudin 1 (see Fig 5) [24]. The
consequences of barrier defects are two-fold; it results in the release of cytokines IL-1a
and IL-1b, which are mediators in a complex inflammatory cytokine cascade that
encourages the leakage of inflammatory cells into the tissue, and contributes to a much
higher susceptibility to developing infections, particularly to Staphylococcus aureus [20].
By preventing uncontrolled water loss from the skin, filaggrin keeps the pH of the skin in
check [20]. Filaggrin is also crucial for the formation of natural moisturizing factor in the
skin [25]. Filaggrin deficiency or defects will therefore interfere with mechanisms that
maintain pH and moisture, making the landscape of the skin less hospitable to permanent
resident commensals. Consequently, these altered conditions could become optimal for
other microbial species that are usually not associated with the ‘normal’ skin
microbiome.
One such species is Staphylococcus aureus. In people without atopic dermatitis, S.
aureus is not a common skin commensal, usually out-competed and driven away by S.
epidermidis, which produces phenol-soluble modulins that specifically exert selective
antimicrobial action on the former [26]. Thus, when barrier defects in the skin make it
inhospitable to S. epidermidis, opportunistic colonization by S. aureus is facilitated. Over
90 percent of patients with atopic dermatitis have S. aureus colonies on their lesional skin
[3]. Therefore, colonization with S. aureus is considered to be an important factor for
atopic dermatitis pathogenesis.
To understand the role of S. aureus in the pathogenesis of atopic dermatitis,
Nakamura et al. performed a series of in vivo and in vitro experiments using mice and
murine samples (Fig 6A-C). The studies were driven by the hypothesis that δ-toxin

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produced by S. aureus acts as a super-antigen that kicks the immune system into
overdrive, wreaking havoc among immune cells in the skin. The primary target of the
δ-toxin was believed to be mast cells, immune cells that produce and store pro-
inflammatory cytokines such as histamine in granules and that then degranulate to release
these cytokines into their local environment when stimulated. In the skin, mast cell
degranulation propagates Th2-type responses, resulting in local epidermal inflammation
[27], a common symptom of atopic dermatitis. Previously unstimulated mast cells were
introduced to δ-toxin derived from S. aureus and ionomycin, a known mediator of mast
cell degranulation. The results supported the hypothesis, showing that δ-toxin causes the
immediate degranulation of mast cells in culture, in a manner similar to that of
ionomycin. Application of δ-toxin directly onto murine skin also showed observable
worsening of disease state [28]. Furthermore, supernatant from cultures of a mutant strain
of S. aureus that lacked the ability to produce δ-toxin caused significantly reduced mast
cell stimulation when compared to the effect brought about by supernatant from cultured
wild-type S. aureus. Colonization of this mutant strain on murine skin was also
associated with a lower degree of mast cell degranulation and, thus, a better disease
status. [28]. This gold-standard study links the presence of δ-toxin producing S. aureus to
higher degrees of mast cell degranulation both in vivo and in vitro and also demonstrates
the consequent worsening of skin disease status in vivo. Replication of this study using
human mast cells for in vitro assays and human in vivo trials of S. aureus strain
colonization would further link the presence of the bacteria to skin inflammation and,
therefore, atopic dermatitis.
Increased S. aureus colonization has also been shown to adversely affect the

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diversity of the microbiome which, in turn, is linked to a worsened disease status [29]. S.
aureus colonization significantly increases and microbiome diversity significantly suffers
during inflammation flares in people with atopic dermatitis [29]. Studies reaching this
conclusion have so far been unable to ascertain whether the observed decrease in
diversity is directly caused by S. aureus or if the defective skin barrier function is the
underlying mechanism that also mediates this. Investigation of murine skin microbiome
development in mice with barrier protein defects or deficiencies could be the first step
towards understanding this conundrum.
Interestingly, S. epidermidis is also observed to become more prevalent during
flares of inflammation which, given the antagonistic nature of S. epidermidis against S.
aureus, was not predicted [29]. However, it is hypothesized that the two Staphylococcus
species may engage in mutualistic relationships under certain conditions such as those of
atopic dermatitis, allowing S. epidermidis to ‘switch allegiances’ when it is conducive to
survival. This might manifest itself in the enhanced common resistance to antimicrobial
peptides [30]. Alternatively, the spikes in S. epidermidis could merely be a compensatory
flare during attempts to control and reign in S. aureus.
BARRIER REPAIR THERAPY
Currently, atopic dermatitis flares are treated by palliative management. This
involves both preventing flares by avoiding known triggers of the condition, regular
moisturizing and general skin care, as well as the reduction of flares with topical
application of corticosteroids [25].

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Due to their association with the regulation of immune responses and
inflammation, corticosteriods have been used for over the past half-century as regular
treatment for atopic dermatitis; they have actually been shown to impair skin barrier
function by dehydrating the skin [31]. Keeping the skin hydrated by topically applying
moisturizers immediately after water absorption (such as after a shower) is crucial to the
management of the condition. Retained dermal water also increases the efficacy of
pharmacological molecules used to manage atopic dermatitis and reduces the dehydrating
effect of corticosterioids [25].
A study examining the diversity of the skin microbiome at base-line, mid-flare,
and post-treatment levels showed a significant decrease in microbiome diversity during
the flare and an increase in diversity after atopic dermatitis treatment began [29]. From
the perspective of the microbiome, atopic dermatitis treatments maintain the diversity of
the microbiome or help increase diversification after a flare by keeping transepidermal
moisture and pH at optimal levels. The lack of antimicrobial agents present in these
atopic dermatitis therapies suggest that the reduction in S. aureus colonization may,
instead, be caused by the return of the skin landscape to its normal form.
Despite this, due to the complex nature of atopic dermatitis, the scientific
community has not conclusively been able to determine whether or not these therapies
are efficacious against atopic dermatitis. Literature reviews seeking to answer this
question have cited studies with results that contradict each other and shown that most
studies are not scientifically rigorous enough due to small sample sizes and very narrow,
focused definitions of ‘efficacy’ (some studies, for example, overlooked clinical efficacy
and focused on the infiltration of inflammatory cells instead) [25].

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THE BIG PICTURE
While much is still unknown about the host-commensal interactions with regards
to dermal immunity, it is clear that microbiome behaviour seems to be governed by the
integrity of the skin barrier function (Fig 7). In the absence of skin barrier defects, the
skin is more hospitable to commensals such as S. epidermidis, which seems to play a
protective role in immune regulation and modulation. However, when the skin barrier is
defective, the skin microbiome population changes for the worse. S. aureus becomes
more prevalent, which causes mast cell degranulation and local inflammation.
FUTURE DIRECTIONS
As we move from an age where non-human cells are indiscriminately seen as
foreign, non-self, harmful and, thus, to be eliminated to improve health outcomes to an
age where we have a better understanding of how the human body is an ecosystem
teeming with cells that are non-human but are neither foreign nor harmful, we have a lot
to learn about how our bodies react to and interact with our microbiomes and the role
these interactions play in health and disease.
With the rise of meta-“omics” technologies, there is a vast world of microbiomes
waiting to be explored. A deeper and more complex understanding of mechanisms that
keep us healthy or make us ill will provide entryways for more innovation. With regards
to the skin microbiome in particular, our improving understanding of the pathogenesis of

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atopic dermatitis can lead to the development of new therapeutics such as those that block
δ -toxin production or supplement defective skin barrier proteins.

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30. Peschel A, Jack RW, Otto M, Collins LV, Staubitz P, Nicholson G, et al.
Staphylococcus aureus resistance to human defensins and evasion of neutrophil
killing via the novel virulence factor MprF is based on modification of membrane
lipids with l-lysine. J Exp Med. 2001;193: 1067-1076.
31. Leung AK, Hon KE, Robson WLM. Atopic dermatitis. Adv Pediatr. 2007;1: 241-
273.

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FIGURE LEGENDS
Fig 1: Simplified flow diagram highlighting key gene sequencing and bioinformatics
techniques used for microbiome profiling. [Human Microbiome Project Consortium,
2012, Figure 1]
Fig 2: Phylogenetic tree illustrating profiled human microbiome. In 2008, the
National Institute of Health began the Human Microbiome Project. This five-year
initiative characterized microorganisms associated with the human body, both
healthy and diseased. The Project found a staggering array of microbial diversity in
association with the human body, with 90% of cells in and on the body of non-
human genetic origin. Despite this, the human microbiome is dominated by four
main phyla: Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria. [Morgan et
al. 2013, Figure 2]
Fig 3: Commensal diversity of human skin varies by body site niche. Qualitative
proportions of bacterial phyla found on various parts of the skin depends on
characteristics of the skin: thickness, moisture levels, density of sebaceous glands,
hair follicle density, and frequency of surface contact/exfoliation. [Grice & Segre,
2011, Figure 3]
Fig 4: Relationship between skin cells and commensals for maintaining immune
function. Both keratinocytes and skin commensals produce antimicrobial peptides
that prevent skin infections. Commensal-derived lipases help break down sebum
produced by sebaceous glands to form free fatty acids, which have been shown to
have antimicrobial properties. The presence of commensal bacteria also directly
enhances the innate immune function of keratinocytes.
Fig 5: Atopic dermatitis skin samples express lower levels of skin barrier proteins.
Immunohistochemical staining for filaggrin, claudins, and IL-17 was done on control
(non-AD) and atopic dermatitis skin samples taken from volunteers via 4mm punch
biopsy. The specimens were scanned and protein expression was calculated by
determining the percentage of sample area positively stained. Counts of IL-17
positive cells were also taken. Control specimens had significantly higher expression
of filaggrin and claudin 1, and significantly lower levels of IL-17 expression. AD
status did not affect the expression levels of claudin 4. [Batista et al. 2014, Figure 1c]
Fig 6: δ-toxin from S. aureus causes localized skin inflammation. A Electromicroscopic
images of unstimulated murine mast cells (Cont), mast cells stimulated with δ-toxin
and ionomycin, a known mast cell stimulant (positive control). Both ionomycin and
δ-toxin caused mast cell degranulation. [Nakamura et al. 2013, Figure 1d] B Mice
were colonized with wild-type S. aureus, ∆hld S. aureus (δ-toxin deficient mutant), or
were treated with PBS (control). Colonization by wild-type S. aureus resulted in
localized redness and epidermal disruption while both the control and ∆hld did not.
[Nakamura et al. 2013, Figure 4c] C Number of neutrophils in skin after colonization

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with wild-type S. aureus, ∆hld S. aureus, or after PBS treatment. Wild-type S. aureus
colonization resulted in significant increase in epidermal neutrophil counts.
Fig 7: Skin barrier integrity governs microbiome membership and skin health.

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FIGURES
Fig 1

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Fig 2

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Fig 3

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Fig 4
COMMENSAL
BACTERIA
KERATINOCYTES
AND SEBACEOUS
GLANDS
AMPs
FREE FATTY
ACIDS

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Fig 5

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Fig 6

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Fig 7